Original ink-jet printers were developed with an ink supply and print head combined into one replaceable unit. This was a good solution for ease of replacement when the ink was completely consumed or the print head developed problems firing properly over time due to various wear-out mechanisms. Any of these problems would result in poor print quality or no printing at all.
As the reliability of the print heads increased to the point at which they could outlive the ink supply portion, printers could be designed with long-lived, permanently installed print heads and replaceable ink supplies. This allowed reduced manufacturing and operating costs as well as improved operating convenience for the printing system. However, due to several physical mechanisms (e.g. air permeability of materials, air ingestion through the nozzles and other components) longer lived print heads have problems with air accumulation over time and usage. Sufficient air may accumulate in areas of the printing system to cause poor print quality or prevent ink from flowing and being printed altogether.